Hydraulic systems, particularly those used in conjunction with an internal combustion engine, have been known for years. For example, Caterpillar Inc. of Peoria, Ill. has been successfully manufacturing and selling hydraulic fuel injection systems for many years. In the past, these systems typically included at least one common rail containing high pressure actuation fluid that was supplied to actuate a plurality of hydraulic devices, such as hydraulically actuated fuel injectors and/or gas exchange valve actuators (engine brake, intake, exhaust). The high pressure common rail was supplied with pressurized actuation fluid by a fixed displacement pump. Control of pressure in the common rail was maintained by sizing the pump to always supply more than the needed amount of high pressure fluid and then utilizing a rail pressure control valve to spill a portion of the fluid in the common rail back to the low pressure reservoir. The control system strategy for these systems typically relied upon a feedback control loop in which the desired rail pressure was compared to the measured or estimated rail pressure, and the position of the rail pressure control valve was set as a function of the error signal generated by that comparison. A system of this type is illustrated, for example, in U.S. Pat. No. 5,357,912 to Barnes et al. While these hydraulic systems, and the control thereof, have performed magnificently for many years, there remains room for improvement.
One area in which these previous hydraulic systems could be improved is by decreasing the amount of pressurized actuation fluid that is spilled back to the low pressure reservoir without performing any useful work, such as actuating one of the hydraulic devices. In other words, energy is consumed and arguably wasted whenever the rail pressure control valve opened to allow pressurized fluid from the high pressure rail to leak back to the low pressure reservoir. In order to decrease the amount of energy consumed in controlling the pressure in the hydraulic system, one strategy has been to introduce a variable delivery pump, and eliminate the previous rail pressure control valve. Such a hydraulic system is shown and described in co-owned U.S. Pat. No. 6,035,828 to Anderson et al. This system greatly reduces the amount of wasted energy since the pump is controlled to produce only the amount of actuation fluid necessary to maintain a desired rail pressure. Although this type of fluid supply and pressurization strategy has considerable promise, it still may suffer from at least one subtle drawback when it is controlled via a feedback loop based upon a comparison of the desired rail pressure to the actual rail pressure. Due at least in part to the fact that the fluid being consumed from the high pressure common rail can be rapidly and continuously changing, engineers have observed that the control system can be at least temporarily overwhelmed in this highly dynamic system. In other words, the system can sometimes demonstrate an inability to both maintain an adequate fluid supply to the hydraulic devices and do so at the desired pressure without unacceptable lags between the control system response and the fluid demands of the hydraulic devices.
Another potential problem area in controlling these hydraulic systems using a variable delivery pump lies in the inevitable fact that each pump has slightly different performance characteristics. These variations in performance can most often be attributed to the geometrical tolerance assigned to the various components that make up the pump. For instance, slight variations in the diametrical clearances between pump pistons and their respective barrels can produce a substantial and even measurable difference in performance from one pump to another. Since the control system often operates under the assumption that the pump is behaving with performance parameters equal to a hypothetical nominal pump, the timing and accuracy of maintaining a desired pressure in the common rail can sometimes be unacceptably large. In other words, the accuracy and timing of producing a desired rail pressure can suffer when the pump deviates in its performance from that of a nominal pump. One possible strategy for dealing with this problem would be to attempt to reduce tolerances in the various components to a level that resulted in pumps having relatively low variability. However, such a strategy may not be viable because of the likely large number of rejected pumps that would fall outside of the accepted variability range and/or potentially costly efforts to reduce component tolerances that would be required to produce pumps with low variability.
The present invention is directed to these and other problems associated with variable delivery pumps and hydraulic systems.